libcamera: Replace toString with operator<<() for geometry classes

Now that geometry classes implement the stream formatting operator<<(),
use it instead of the toString() function.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/v4l2_videodevice.cpp

@@ -433,7 +433,7 @@ bool V4L2BufferCache::Entry::operator==(const FrameBuffer &buffer) const
 const std::string V4L2DeviceFormat::toString() const
 {
 	std::stringstream ss;
-	ss << size.toString() << "-" << fourcc.toString();
+	ss << size << "-" << fourcc.toString();
 	return ss.str();
 }